Let core-16 mixin do the work for mobile view

- Removes media query for `max-width 320px` view, as core 16 will reduce to 14px based on the viewport
  - https://github.com/alphagov/govuk_frontend_toolkit/blob/master/stylesheets/_typography.scss#L85-L90
- Also removes `article td small: 12px` as there appears to be application layer css that covers this on pages like: https://www.gov.uk/vehicle-tax-rate-tables
- Tables default view will be core-16 with nta-tabular-numbers. ( ˘ ³˘)